In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as those associated with boiler systems for providing steam to a power plant, a hot process gas (or flue gas) is generated. Such a flue gas will often contain, among other things, carbon dioxide (CO2) The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognised, and have resulted in the development of processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels. One such system and process has previously been disclosed and is directed to a single-stage Chilled Ammonia based system and method for removal of carbon dioxide (CO2) from a post-combustion flue gas stream.
Known Chilled Ammonia based systems and processes (CAP) provide a relatively low cost means for capturing/removing CO2 from a gas stream, such as, for example, a post combustion flue gas stream. An example of such a system and process has previously been disclosed in pending patent application PCT/US2005/012794 (International Publication Number: WO 2006/022885/Inventor: Eli Gal)), filed on 12 Apr. 2005 and titled Ultra Cleaning of Combustion Gas Including the Removal of CO2. In this process the absorption of CO2 from a flue gas stream is achieved by contacting a chilled ammonia ionic solution (or slurry) with a flue gas stream that contains CO2.
FIG. 1A is a diagram generally depicting a flue gas processing system 15 for use in removing various pollutants from a flue gas stream FG emitted by the combustion chamber of a boiler system 26 used in a steam generator system of, for example, a power generation plant. This system includes a CO2 removal system 70 that is configured to remove CO2 from the flue gas stream FG before emitting the cleaned flue gas stream to an exhaust stack 90 (or alternatively additional processing). It is also configured to output CO2 removed from the flue gas stream FG. Details of CO2 removal system 70 are generally depicted in FIG. 1B.
With reference to FIG. 1B, CO2 removal System 70 includes a capture system 72 for capturing/removing CO2 from a flue gas stream FG and a regeneration system 74 for regenerating ionic solution used to remove CO2 from the flue gas stream FG. Details of capture system 72 are generally depicted in FIG. 1C.
With reference to FIG. 1C a capture system 72 of a CO2 capture system 70 (FIG. 1A) is generally depicted. In this system, the capture system 72 is a chilled ammonia based CO2 capture system. In a chilled ammonia based system/method for CO2 removal, an absorber vessel is provided in which an absorbent ionic solution (ionic solution) is contacted with a flue gas stream (FG) containing CO2. The ionic solution is typically aqueous and may be composed of, for example, water and ammonium ions, bicarbonate ions, carbonate ions, and/or carbamate ions. An example of a known CAP CO2 removal system is generally depicted in the diagram of FIG. 1C.
With reference to FIG. 1C, an absorber vessel 170 is configured to receive a flue gas stream (FG) originating from, for example, the combustion chamber of a fossil fuel fired boiler 26 (see FIG. 1A). It is also configured to receive a lean ionic solution supply from regeneration system 74 (see FIG. 1B). The lean ionic solution is introduced into the vessel 170 via a liquid distribution system 122 while the flue gas stream FG is also received by the absorber vessel 170 via flue gas inlet 76.
The ionic solution is put into contact with the flue gas stream via a gas-liquid contacting device (hereinafter, mass transfer device, MTD) 111 used for mass transfer and located in the absorber vessel 170 and within the path that the flue gas stream travels from its entrance via inlet 76 to the vessel exit 77. The gas-liquid contacting device 111 may be, for example, one or more commonly known structured or random packing materials, or a combination thereof.
Ionic solution sprayed from the spray head system 121 and/or 122 falls downward and onto/into the mass transfer device 111. The lean ionic solution feeding to the spray head system 122 and the recycled ionic solution feeding to spray head 121 can be combined and sprayed from one spray header. The ionic solution cascades through the mass transfer device 111 and comes in contact with the flue gas stream FG that is rising upward (opposite the direction of the ionic solution) and through the mass transfer device 111.
Once contacted with the flue gas stream, the ionic solution acts to absorb CO2 from the flue gas stream, thus making the ionic solution “rich” with CO2 (rich solution). The rich ionic solution continues to flow downward through the mass transfer device and is then collected in the bottom 78 of the absorber vessel 170. The rich ionic solution is then regenerated via regenerator system 74 (see FIG. 1B) to release the CO2 absorbed by the ionic solution from the flue gas stream. The CO2 released from the ionic solution may then be output to storage or other predetermined uses/purposes. Once the CO2 is released from the ionic solution, the ionic solution is said to be “lean”. The lean ionic solution is then again ready to absorb CO2 from a flue gas stream and may be directed back to the liquid distribution system 122 whereby it is again introduced into the absorber vessel 170.
After the ionic solution is sprayed into the absorber vessel 170 via spray head system 122, it cascades downward onto and through the mass transfer device 111 where it is contacted with the flue gas stream FG. Upon contact with the flue gas stream the ionic solution reacts with CO2 that may be contained in the flue gas stream. This reaction is exothermic and as such results in the generation of heat in the absorber vessel 170. This heat can cause some of the ammonia contained in the ionic solution to change into a gas. The gaseous ammonia then, instead of migrating downward along with the liquid ionic solution, migrates upward through the absorber vessel 170, along with and as a part of the flue gas stream and, ultimately, escaping via the exit 77 of the absorber vessel 170. The loss of this ammonia from the system (ammonia slip) decreases the molar concentration of ammonia in the ionic solution. As the molar concentration of ammonia decreases, so does the R value (NH3-to-CO2 mole ratio).
When a flue gas stream is contacted with the ionic solution, the carbon dioxide contained in the flue gas stream reacts to form bicarbonate ion by reacting with water (H2O) and with hydroxyl ion (OH−). These “capture reactions” (Reaction 1 through Reaction 9, shown below) are generally described as follows:CO2(g)→CO2(aq)  (Reaction 1)CO2(aq)+2H2O→HCO3−(aq)+H3O+  (Reaction 2)CO2(aq)+OH−→HCO3−(aq)  (Reaction 3)
The reactions of the NH3 and its ions and CO2 occur in the liquid phase and are discussed below. However, in low temperature, typically below 70-80 F and high ionic strength, typically 2-12M ammonia ions the bicarbonate produced in Reaction (2) and Reaction (3), reacts with ammonium ions and precipitates as ammonium bicarbonate when the ratio NH3/CO2 is smaller than 2.0 according to:HCO3−(aq)+NH4+(aq)→NH4HCO3(s)  (Reaction 4)
Reaction 2 is a slow reaction while Reaction 3 is a faster reaction. At high pH levels such as, for example when pH is greater than 10, the concentration of OH− in the ionic solution is high and thus most of the CO2 is captured through reaction (3) and high CO2 capture efficiency can be achieved. At lower pH the concentration of the hydroxyl ion OH− is low and the CO2 capture efficiency is also low and is based mainly on reaction (2).
In the Chilled Ammonia Based CO2 Capture system(s)/method(s) the CO2 in the flue gas stream is captured by contacting the flue gas stream with an aqueous ammonia solution allowing the CO2 in the flue gas stream to directly react with the aqueous ammonia. At low R, typically less than about 2, and pH typically lower than 10, the direct reaction of CO2 with ammonia contained in the ionic solution is the dominant mechanism for CO2 capture. The first step in the CO2 sequence capture is the CO2 mass transfer from the gas phase to the liquid phase of reaction (1). In the liquid phase a sequence of reaction occur between the aqueous CO2 and aqueous ammonia:CO2(aq)+NH3(aq)→CO2*NH3(aq)  (Reaction 5)CO2*NH3(aq)+H2O→NH2CO2−(aq)+H3O+  (Reaction 6)NH2CO2−(aq)+H2O→NH4+(aq)+CO3=(aq)  (Reaction 7)CO3=(aq)+NH4+(aq)→HCO3−(aq)+NH3(aq)  (Reaction 8)CO3=(aq)+H3O+→HCO3−(aq)+H2O  (Reaction 9)
As described above the bicarbonate produced in Reaction (8) & Reaction (9) can react with ammonium ions to precipitate as solid ammonium bicarbonate based on Reaction (4), while the ammonia produced in Reaction (8) can react with additional CO2 based on Reaction (5).
The sequence of the chain of reactions (5) through (9) is relatively slow and thus requires a large and expensive CO2 capture device. The slow rate of CO2 absorption is due to: 1) one or more slow reactions in the sequence of capture reactions (Reaction 1 thru Reaction 9); and 2) the accumulation of intermediate species, such as CO2*NH3 and NH2 CO2−, in the ionic solution. The accumulation of intermediate species slows the CO2 capture process and results in lower CO2 capture efficiency with a power generation facility. Thus, a heretofore unaddressed need exists in the industry to accelerate the rate of the CO2 capture reactions that allows significant reduction in the size and thus the cost of the CO2 capture device and its auxiliary systems.
Further, features of the present invention will be apparent from the description and the claims.